The present invention relates to a stencil printer or similar printer and more particularly to a printer including a plurality of spaced print drums for producing a color print by passing a paper sheet only once.
A color stencil printer, for example, includes a plurality of print drums sequentially arranged at preselected intervals in a direction in which a paper sheet is conveyed. While a paper sheet is conveyed in the above direction, the upstream print drum to the downstream print drum, in the above direction, each transfer an ink image of particular color to the paper sheet. As a result, ink images of different colors are transferred to the paper sheet one above the other. The printer can therefore produce a color print by passing the paper sheet only once. Such a single-pass system is far more efficient than a system that requires a print drum to be replaced color by color and repeatedly feeds the same paper sheet. The single-pass system, however, has some problems left unsolved due to a short interval between consecutive printing positions, as will be described hereinafter.
An image transferred from an upstream drum assigned to, e.g., a first color to a paper sheet arrives at a downstream print drum assigned to, e.g., a second color before ink forming the image dries. As a result, the ink is transferred to a master or cut stencil wrapped around the downstream print drum and then transferred from the master to the next paper sheet. As for the first paper sheet, the wet ink of the first color is simply transferred to the mater existing on the print drum of the second color and does not matter at all. However, the ink of the first color is again transferred from the master to the second paper sheet carrying an image of the first color transferred from the print drum of the first color (generally referred to as retransfer).
Retransfer superposes ink of the same color and therefore does not degrade image quality so long as the image transferred from the master to the second paper sheet is in accurate register with the image existing on the second paper sheet. However, if the retransferred image is shifted from the image existing on the paper sheet, it produces a shadow or so-called offset ghost. For a given amount of shift, the offset ghost renders a thick line blurred and renders a thin line double, thereby degrading image quality to a critical degree.
While retransfer is not avoidable in the single-pass color printing system, an offset ghost ascribable to positional shift can be accurately reduced if the upstream and downstream drums are accurately synchronized to each other during rotation and if a paper sheet is accurately conveyed. Stated another way, should the print drums fail to rotate in synchronism with each other, an offset ghost repeatedly occur.
To reduce offset ghosts, it has been customary to interlock the upstream and downstream print drums with respect to drive. Japanese Patent Laid-Open Publication No. 4-329175, for example, discloses an arrangement wherein the shafts of print drums are connected by gears. Japanese Patent Laid-Open Publication No. 7-17121 teaches an arrangement wherein the shafts of print drums are connected by a timing belt. With the gear scheme, it is possible to reduce the amount of an offset ghost by increasing the precision of the gears. High precision gears, however, increase the production cost of the printer. By contrast, the timing belt scheme successfully reduces the production cost because use can be made of timing pulleys and other inexpensive members that can be produced by, e.g., injection molding on a quantity basis. This kind of scheme, however, increases the amount of an offset ghost more than the gear scheme.
Technologies relating to the present invention are also disclosed in, e.g., U.S. patent application Ser. Nos. 09/079,287, 09/164,372, 09/274,324 now U.S. Pat. No. 6,205,918 and 09/532,055.